Copy rotation does not always rotates correctly
It always works correctly.  It does not always work like you'd expect.  The differences are basically down to how Euler angles operate, especially in conjunction with constraints.
So when you copy rotation, here's what it does:

It decomposes your target bone's orientation into XYZ Euler triplets.  These are not necessarily or even likely to be the same XYZ triplets you keyframed for that bone.  That is because Euler angles are aliased: there is more than one set of Euler angles that corresponds to any particular orientation.  But in order to accommodate space-switching, we have to get only a single set of numbers out, so there's some information that's lost.

If you want to see what these anti-aliased (not a good thing in this case) numbers are, you can duplicate the control bone, give it a copy transforms constraint targeting the original, and then apply visual transform (or bake animation.)  You will see that the transforms you get are probably not the ones you expected.

We multiply the components of that triplet by the influence of the constraint, and then rotate the constrained bone with it.  Changing influence does not give you rotation along a plane-- rotation in a single axis.  That is, there's torque.  That's because Euler angle rotation doesn't interpolate in a straight line.

Those two things combined mean that copy rotation, particularly at influence &lt;1.0, particularly for 2-axis copy rotation, doesn't work like people intuit it ought to.  Copy rotation is part of a set (with limit rotation) of the least intuitive constraints, because of how people think about 3D angles working, and the differences between that and how they actually work.
If only the first thing is a problem, you could consider using a driver instead of a constraint.  If you don't need space switching, drivers can let you work on raw, no-lost-data f-curve numbers.
However, what I'd generally say is, try to find a different way to do what you want.  Copying rotation of 3 axes, at influence 1.0, is perfectly safe.  Damped tracking and locked tracking with influence less than 1.0 is safe, and operates in the &quot;swing angle&quot; (smallest rotation) rather than with any torque.  One way that should get you what you want is to copy all rotation fully, and then restore a part of it by damped and locked tracking non-deforming marker bones.  That may be more complicated than what you actually need though.